Hughes/ Milltech Motorized Waveguide Switch Control --- Comment by Bruce, KG6OJI

The recent discussion about Hughes waveguide switches peaked my curiosity and led me to breadboard a TTL controller with relay voltage doubler for 12 VDC field operation of my switch. The one I have was part of a Hughes 4772XH sweep system. In the process I wound up opening the housing to check its internal construction. One finding was that although the switches from three-head sweep units can serve as manual four-position switches, the position indications of these (at least mine) are set up for electrical operation in only three positions. Presumably, the products sold as stand-alone switches were equipped for four-position operation.


Position sensing relies on an aluminum disk with four pairs of diametrically opposite 0.073-inch diameter holes, each pair on a different radius. These allow the switch to move to the nearest of two equivalent 180-degree positions rather than traveling all of the way around. A U-shaped over-and-under assembly with three subminiature LEDs, each driven from +5 V through a 470-ohm resistor is on the top, and three subminiature photo-diodes, imbedded in a small circuit board, is on the bottom. The common sides of the photo-diodes are grounded. The active photo-diode terminals are routed to a DM54L04W hex inverter in a 14-lead ceramic flat pack.


DM54L04W Pin

1 n.c. _ 8 orange wire
2 n.c. _ 9 middle photo-diode
3 n.c., lead cu _ 10 red wire
4 +5V white wire _ 11 ground -- black wire
5 n.c. lead cut _ 12 brown wire
6 n.c. _ 13 outer photo-diode
7 inner photo-diode _ 14 leads to thorugh-hole
but apparently n.c.



Although I did not confirm this, I assume the brown wire is for switch position 1 and runs to pin 5 of the Lemo connector, the red position 2 and runs to pin 6, and the orange position 3 and runs to pin 7. Similarly, the white +5 V wire should run to pin 1 and the black ground wire to pin 8.

With the housing slid off, the connections to the stepping motor are clearly visible. There are no diodes or other components associated with the motor. The motor wires are the heavier red that runs to pin 3 and black that runs to pin 4 of the connector. Connections at the cable Lemo connector cannot be easily inspected because they are enclosed in an outer heat-shrink sleeve.

The top cap, which also serves as a knob to manually rotate the switch, is retained with three Allen setscrews, set 90-degrees apart. These require an allen wrench 1/16-inch across the flats. To remove the knob the switch should be rotated to position 2 so that the center of the three small upper access holes is facing the front or waveguide output of the switch. The front and right-hand setscrews can then be loosened through front and right access holes in the casing. Access to the left setscrew is blocked by the internal position sensing assembly. If the cap cannot be lifted free, the knob should be rotated 90-degrees counter-clockwise to allow the third screw to be accessed through the front hole. Note that the central shaft has a flat for the central setscrew that will be facing the front with the switch in position 2.

I found the sweep head assembly has a previously undiscovered 10-ohm, 10-watt Dale resistor and another component that appears to be a thermostat mounted under a horizontal support bar of the assembly. It is difficult to properly inspect these components, because of limited access, but I suspect the resistor and an accompanying thermostat may serve as a heater to maintain a minimum temperature for frequency stability of the sweep head. These components are fed from the relay control cable with RG-174 coax that breaks out of the loom. The center conductor likely connects to Pin 2 of the Lemo connector and the outer shield to pin 8. Interestingly, there was no connection to Pin 2 on the chassis connector of the Hughes 4772xH driver assembly I disassembled. That connector pin showed no sign of solder.

The stepping motor within the housing is a Ledex SQIL? Series, model 8481D stepping motor marked duty, 24 steps, 24 VDC, 3.11 A, 216624-026. I was surprised to find these switches driven by Ledex solenoids. As Ledex is still in business, perhaps they can supply a mechanically compatible two-position replacement that would rotate the necessary 45-degrees with either spring or power return. I did not try to disassemble the drive mechanism, but suspect the mechanical indexing is Hughes made. It requires considerable torque to overcome as you can feel by turning the selector knob.

My implementation incorporates two TTL chips, a 555 timer, and a relay voltage-doubler for 12 VDC operation. It works well over an input voltage range of 10 to 14 volts, operating at nine steps per second. Should anyone be interested, I would make a sketch.


Updated 9/30/2017